Blood obtained for lab draws are centrifuged for 15 minutes at a speed of 3,000 g, and at a temperature of around 18ºC. Centrifugation is performed within 1 hour after sample collection. The plasma samples are then analyzed in batches for anti-factor Xa activity by amidolytic assay, done at noon daily in Long Beach Memorial's Department of Pathology (Coagulation). If the assay is not performed within 30 minutes of centrifugation, the plasma is frozen. Plasma samples may be frozen for up to 6 months. The minimum detectable limit of this assay is 0.1 U/ml, and the linearity range extends to 2.0 U/ml. Therapeutic anti-factor Xa activity is designated as 0.5-1.2 U/ml. The coefficient of variation for this assay is 98%.13